Question

a= -15 ,d= 2 ,Sn= -55. then n=?​

Answer 1

a = -15

d = 2

sn = -55

n = ?

sn = n / 2 ( a + an )

-55 = n / 2 ( -15 + ( -15 ) + ( n -1 ) 2

-55= n / 2 ( -30 + 2n - 2 )

-55 = n / 2 ( -32 + 2n )

-55-32 = n / 2 + 2n

-87 = n / 2 + 2n

-87 × 2 = n + 2n

- 174 = 3n

- 174 / 3 = n

- 58 = n

Answer 2

Answer:

a= -15

d=2

Sn= -55

n=?

Sn=n/2×(a+an)

-55=n/2×(-15+a+(n-1)×d)

-55= n/2×(-15+-15+(n-1)2)

-55×2/n=-15-15+2n-2

-110/n= -30+2n-2

-110/n= -32+2n

-110/n+32=2n

(-110×1+32×n)/n=2n

(-110+32n)/n=2n

cross multiplication

-110+32n=2n²

prime factorization:

=2n²-32n+110=0

=2n²-22n-10n+110=0

2n(n-11)-10(n-11)=0

(2n-10) (n-11)=0

2n-10=0 or n-11=0

2n=10 or n=11

n=10/2. or. n=11

n=5 or n=11